It is well-known to manufacture xylenes by the alkylation of toluene and/or benzene with methanol, and in particular to selectively make paraxylene (PX) product using zeolite catalyst. See, for instance, U.S. Pat. Nos. 4,002,698; 4,356,338; 4,423,266; 5,675,047; 5,804,690; 5,939,597; 6,028,238; 6,046,372; 6,048,816; 6,156,949; 6,423,879; 6,504,072; 6,506,954; 6,538,167; and 6,642,426. See also more recently U.S. application Ser. No. 13/557,605, and references cited therein. Paraxylene selectivity is highly sought after because of the economic importance of paraxylene relative to meta- and orthoxylene. Although each of the xylene isomers have important and well-known end uses, paraxylene is currently the most economically valuable, serving as an intermediate in such important and diverse end uses as bottle plastic and polyester fibers.
One of the problems with xylenes streams produced by alkylating aromatic species such as benzene and/or toluene with alkylating agents such as methanol and/or dimethyl ether over solid catalysts, such as in the aforementioned processes, is the product stream may contain styrene impurities. This has recently been observed and set forth in Provisional Patent Applications 61/711,341 and 61/681,486. Styrene impurities can cause operability problems for downstream process, for example paraxylene recovery by adsorptive separation processes, e.g., Parex™ Process or Eluxyl™ Process, as well as other processes used to take paraxylene to end products, such as in processes used to make purified terephthalic acid/anhydride and subsequent steps to making fibers or bottle plastic therefrom.
One method of removing styrene is to convert said species to ethylbenzene by selective hydrogenation. Several characteristics of purifying xylenes containing styrene impurities make selective hydrogenation of styrene challenging. It is highly desirable to minimize the ring saturation reactions since separation of dimethylcyclohexane, ethylcyclohexane, and/or other saturated C8 hydrocarbons from xylenes is difficult. Another potential challenge is that the desired product, paraxylene, is present at higher-than-equilibrium concentration. The catalyst used to hydrogenate styrene must therefore show minimal xylenes isomerization activity.
It is known to hydrogenate certain aromatic species in paraxylene enriched streams. See, for instance, U.S. application Ser. Nos. 61/604,926, 61/496,262, 13/303,855, and 13/449,758.
The present inventors have surprisingly discovered a method of selectively hydrogenating styrene impurities present in a xylenes stream nearly stoichiometrically using a catalyst comprising at least one metal selected Groups 8-10 of the Periodic Table, optionally further comprising promoters and/or supports.